Recently, an optical recording medium utilizing a semiconductor laser has been put to practical use. Since the wave length radiated from a semiconductor laser is in the range of near infrared ray at the present time, it is inevitable to use a material which strongly absorbs the near infrared ray for recording or reading with ray semiconductor laser.
As for one representative example of an organic dye absorbing the near infrared ray, there are mentioned naphthalocyanines derivatives. Since the naphthalocyanine having Cu, Co, Ni, Zn, Vo, Mg, Al, Mn, In, Ge or hydrogen as a central element of the naphthalocyanine ring generally have poor solubility in an organic solvent, it has been necessary to use a dry process such as a vacuum vapor deposition method for forming a thin layer thereof (Mol. Cryst. Liq. Cryst., 112, 345-358, 1984).
To improve the solubility thereof, there is proposed a method to introduce an alkyl group into the naphthalocyanine ring as a substituent (J. Chem. Soc., Perkin Trans. 1, (8), 2453-2458, 1988, Japan Laid Open Patent Application 60-184565, ibid 61-25886). This method however, has some disadvantages such as
(i) many steps are required to synthesize the naphthalocyanine derivative, and PA1 (ii) an association of the naphthalocyanine derivative occurs in a solvent at high concentration and the strength of absorbance at the near infrared region decreases thereby (Japan Chemical Society, 57th Autumn Conference, preprint, 1D414, P.642). PA1 (i) the concentration of the naphthalocyanine in the naphthalocyanine/polymer thin film can not be increased, PA1 (ii) the phase-separation between the naphthalocyanine and the polymer occurs at high temperature, and PA1 (iii) the variety of the polymer used in this method is limited.
As for other proposals to improve the solubility thereof, there is proposed a method wherein a polyvalent metal atom having 3 or more valence such as silicon and germanium is used as the central metal atom of the naphthalocyanine and a binding site of the polyvalent metal atom which is not used for binding with the naphthalocyanine skeleton is bound with an alkali group or aryl group (J. Am. Chem. Soc., 106, 7404-7410, 1984, U.S. Pat. No. 4,725,525). Of these, the especially preferred central metal is silicon. But when the naphthalocyanine which is substituted at its central metal is mixed with a polymer capable of forming a film and then used to form a thin film, there appears a tendency for phase-separation to occur between the naphthalocyanine and the polymer in accordance with the concentration increase of the naphthalocyanine or with the lapse of time.
Accordingly, there is further proposed a method wherein a transition metal or Al, Ge, Sn, Ga or In is used as the central metal of the naphthalocyanine and the metal is coordinately bound with a polymer capable of being bound coordinately, whereby to avoid the phase separation between the naphthalocyanine and the polymer (Japan Laid Open Patent Application 61-232448).
This method has disadvantages such as
On the other hand, there is proposed a naphthalocyanine/polymer thin layer in which the naphthalocyanine is covalently bound to the polymer (Japan Laid Open Patent Application 61-177288). This material is made, by using dihydroxysilicon naphthalocyanine as a starting material, through a two step reaction as shown in the following reaction equation; ##STR2## wherein Nc represents naphthalocyanine skeleton; SiNc(OH).sub.2 represents dihydroxysilicon naphthalocyanine; (CH.sub.3).sub.2 SiCl.sub.2 represents dimethyldichlorosilane; ROH represents polymer having hydroxy groups
These soluble naphthalocyanines may be coated to form a highly reflective layer, and a pit can be formed by radiation of a laser beam. Utilizing such property, there have been proposed many writable optical recording media using a soluble naphthalocyanine which is recorded by forming a pit. Although the above naphthalocyanine dye which is substituted with a hydrophobic group such as an alkyl group and aryl group can be formed into a layer as it is, it is not economical to use the dye as it is for the recording medium because the dye is very expensive. The application of the naphthalocyanine dye mentioned above is limited to a highly hydrophobic polymer because of its hydrophobicity when it is intended to use the dye by mixing with the another polymer Further, since this dye is easily sublimated and has relatively low heat-durability among the naphthalocyanine compounds, the record of the optical recording medium may be damaged even with a low energy level laser beam during the reading process.